A storage battery pack, having a plurality of secondary batteries (cells) connected in series, which has an electronic circuit to average cell voltages of the cells, has been known. As for averaging cell voltages, an active method, in which electricity is transferred between the cells, is gathering attention.
An electronic circuit adopting the active method has a transformer and a switching element for activating the transformer, accumulates electricity in a primary coil during the time the switching element is turned on, and outputs electricity accumulated in the primary coil to a secondary coil when the switching element is turned off. An averaging of cell voltages in secondary batteries is performed by repeating such an operation to provide a cell connected with the secondary coil with electricity, in an electronic circuit adopting the active method (for example, Japanese Laid-open Patent Publication No. 2002-223528, No. 2011-83182, No. 2013-13268).